EVOLUTION OF LOWER CRUST BENEATH THE HONSHU ARC

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Felsic to mafic plutonism took place in the Honshu Arc from Cretaceous to late Cenozoic. Cenozoic volcanic rocks occurring in the Honshu Arc contain lower crust-derived mafic rocks and upper mantle-derived ultramafic rocks as xenoliths. As to these igneous rocks and xenoliths, we have carried out petrological, petrochemical (major and minor elements) and geochronological studies. According to these studies, the following facts were clarified.1. Western part of Tanakura Tectonic Line of Honshu Arc. (1) Southwest Honshu Arc (Ryoke Belt, a part of San'yo Belt, eastern part of San'in Belt), Northeast Honshu Arc (Ashio & Tsukuba Mountains) : The lower crust beneath these areas was formed from continental lithospheric mantle at late Paleozoic to early Mesozoic, and has tholeiitic chemical composition. The Cretaceous to early Neogene igneous rocks occurring in these areas were formed from these lower cust and/or upper mantle. (2) Southwest Honshu Arc (Kyushu, western part of San'in Belt), Nor … More theast Honshu Arc (Sado Island, Asahi Mountains) : The lower crust and upper mantle beneath these areas have different features and history from those of area "(1)". (3) Middle area between (1) and (2) : The lower crust and upper mantle of two different types, "(1)" and "(2)", are mixed beneath the middle area between (1) and (2). Sr and Nd isotopic features of ingeous rocks occurring in this area are in the middle of those of igneous rocks from areas (1) nad (2).II.Eastern part of Tanakura Tectonic Line of Honshu Arc. History and features of the lower crust and upper mantle beneath this area are probably different from those beneath area "I". Most of the igneous rocks in this area were formed from these materials. However, granitic rocks in the most eastern part of Kitakami Belt were partial melting products of subducted oceanic plate.III.Middle Miocene to Recent. Lower crust and upper mantle beneath the Japan Sea and its surrounding area were newly reconstructed in connection with opening of the Japan Sea. Middle Miocene to Recent igneous rocks of theJapan Sea side were derived from these materials. Less